(a) Field of the Invention
The present invention is concerned with an electronic musical instrument which produces a musical sound by computing the waveshape of a musical tone, and in particular, it pertains to an improvement in the computing system of musical tone waveshape.
(B) Brief Description of the Prior Art
A musical sound produced by a natural musical instrument, in general, represents a composite of a number of tone partials. Accordingly, in order to produce, by an electronic musical instrument, a musical sound resembling that of a natural musical instrument, there has to be formed a composite waveshape, i.e. a musical tone waveshape, of a number of different frequency components, i.e. tone partial components. As the above-said system for forming a musical tone waveshape, there are, roughly speaking, the following two kinds of systems. One of them is a system of synthesizing a musical tone waveshape out of the output signals of a number of oscillators. The other one is the system for obtaining a musical tone waveshape through computation. The former of the systems requires a number of oscillators, so that the electronic musical instrument which adopts the system tends to become complicated in structure and to become expensive. Thus, this system is not suitable for a production of a musical tone waveshape which is comprised of a number of tone partial components. The latter of the systems, on the other hand, has the possibility that an arbitrary musical tone waveshape can be obtained by a relatively simple means by a mere adequate selection of the method of computation.
A known typical technique of the system of computing a musical tone waveshape referred to above is disclosed in U.S. Pat. No. 3,809,786 entitled "COMPUTOR ORGAN". This known technique is such that a musical tone waveshape which is comprised of a number of harmonic components is computed in accordance with a discrete Fourier algorithm. During the respective sampling intervals of the musical tone waveshape which is to be produced, the sampling values of these respective harmonic components are computed at a high speed on time-division basis. The results of the computation are accumulated and thereby the value of sampling of the desired musical tone waveshape is obtained.
This known technique described just above, however, involves the following problem. That is, in case the number of the harmonic components which constitute a musical tone waveshape is large, the speed of calculation has to be increased to a very high degree, and accordingly there is required a device which is capable of conducting a remarkably high speed operation, in order to provide a computing means for use in the computation of the waveshape. In other words, the number of the tone partials of a musical sound which can be produced is subjected to limitation by the inherent operation speed of the computing device employed. Also, in view of the fact that a musical tone waveshape is computed in accordance with Fourier algorithm, it is difficult to produce a musical tone waveshape containing non-harmonic tone partial components.